This invention relates generally to a second stage regenerator suitable for use in a multistage cryogenic refrigerator.
A refrigeration cycle generally referred to as the Gifford-McMahon cycle is disclosed in U.S. Pat. No. 2,906,101. The cycle is further shown embodied in a two-stage refrigerator in a later U.S. Pat. No. 3,312,072. This two-stage arrangement has found relatively wide use in a number of different cryogenic applications. In the two-stage configuration, a pair of different size expansion chambers are utilized to process a working substance in the form of helium gas to attain extremely low temperatures. Each chamber houses a sealed displacer that is able to slide axially in the chamber to vary its volume. The helium is initially compressed to a high pressure and is cycled through the chambers under the control of a rotary valve.
The expansion chambers, which generally define the two stages of refrigeration, are interconnected by a refrigerant flow circuit containing a low temperature heat exchanger and second stage regenerator unit. The regenerator is typically a small, high efficiency unit that is operatively connected between the first stage expansion chamber and the low temperature heat exchanger. The pressure loss over the regenerator is minimized so that the pressure in both chambers remains substantially equal throughout the cycle. The unit is normally housed within a sleeve and is packed with fine lead shot. The lead shot, which can retain its specific heat at low temperatures, serves to remove the heat of refrigeration from the helium gas as it moves in one direction toward the second stage and to give back the heat to gas as it moves in the opposite direction.
The sleeve and the regenerator housing that is enclosed therein are fabricated from different materials. As a consequence, the two members expand and contract at different rates when exposed to the large temperature changes (295.degree. K. -7.degree. K.) that take place in this critical region. Any thermal displacement of the regenerator within the sleeve, no matter how slight, can cause unwanted heat producing friction to be developed in the low temperature region and also upset the volumetric relationship between stages. In any event, unwanted movement of the regenerator within the sleeve will adversely affect the performance of the refrigerator as well as its operating efficiency.
To prevent the second stage regenerator from moving in assembly, it has heretofore been the practice to press fit the regenerator housing tightly into the sleeve using an interference fit. It has been found, however, that under even an extremely tight fit, the regenerator can free itself from the sleeve under actual working conditions and begin to move axially in the sleeve in response to the movement of refrigerant therethrough. Part of the problem resides in the fact that the regenerator housing, which is made from a glass woven fabric is initially expanded outwardly in a radial direction by the tightly packed shot. After being exposed to a number of refrigeration cycles, the shot is redistributed in the housing and the housing is thus permitted to contract back to its normal size. The effectiveness of the press fit is thus lost whereupon the regenerator is able to work itself loose from the retaining sleeve. Beyond unwanted displacement of the unit, this can also destroy the seal between the unit and the sleeve whereupon refrigerant can "blow by" the regenerator causing further problems.
From a manufacturing standpoint, press fitting the second stage regenerator into the enclosing sleeve creates further problems. Because of the tightness of the fit, extreme care must be taken when assembling or disassembling the unit to insure that its component parts are not broken or damaged. The procedures involved in assembling and/or disassembling a unit are complex, time consuming and costly. It is difficult to ascertain if the regenerator unit is properly positioned in relation to the low temperature heat exchanger during assembly and there is some likelihood that an unwanted gap might be left between the two units which will alter the geometry of the system and adversely affect performance and efficiency. Because of the lack of readily interchangeable parts, the replacement of a regenerator unit must be performed by a highly qualified technician.